1. Field of the Invention
The present invention generally relates to an X-ray analysis apparatus for analyzing specimens by irradiating X-rays and, more particularly, to the X-ray analysis apparatus of a type utilizing an artificial multi-layered grating.
2. Description of the Prior Art
The X-ray analysis is currently carried out by using either an X-ray diffraction technique or a fluorescent X-ray technique. In the practice of the X-ray diffraction analysis, substantially collimated X-ray are irradiated to a specimen to be analyzed while the specimen is rotated in one direction, and the X-rays diffracted from the specimen are then detected for calculation according to Bragg's law to determine the spacing of lattice planes thereby to analyze the crystal structure of the specimen. On the other hand, X-ray fluorescence analysis is to measure the fluorescent X-rays which are emitted from a specimen by means of irradiation of primary X-rays generated from an X-ray tube.
The total reflection fluorescent X-ray analysis, which is a kind of the X-ray fluorescence analysis techniques, is carried out by irradiating the specimen with a first-order X-ray so as to be incident on a surface of the specimen at an incident angle so small as to result in a total reflection of the X-ray beams and then detecting fluorescent X-rays generating from the specimen for the determination of elements comprising the specimen. The Japanese Laid-open Patent Publication No. 63-78056 published in 1988, for example, discloses the utilization of this total reflection fluorescent X-ray analysis for the detection of impurities sticking to a surface of the specimen.
In the practice of any one of these X-ray analysis methods, in order to reduce an error in analytical measurement, a curved analyzing element is utilized to provide monochromatic X-rays for irradiation to the specimen to be analyzed. It has, however, been found that, when the source X-rays are passed through the analyzing crystal to provide the monochromatic X-rays, the intensity of the X-rays tends to be lowered to such an extent that no reduction of the analytical error be longer obtained. This will be discussed in more detail for each case with the X-ray diffraction analysis and the total reflection fluorescent X-ray analysis.
FIG. 11 illustrates schematically the prior art X-ray diffraction analysis apparatus.
Referring now to FIG. 11, reference numeral 50 represents an analyzing crystal or a monochromator. This analyzing crystal 50 has a reflective surface 51 used to diffract X-ray beams B1 generating from an X-ray radiation source P, to provide first monochromatic X-ray beams B2 which are subsequently incident on a specimen 2. The first monochromatic X-ray beams are, after having been once converged at an X-ray collecting point Q, diverged again before they impinge upon the specimen 2. A X-ray detector 3 is utilized to detect second monochromatic X-ray beams B6 which have been reflected from the specimen 2. The specimen 2 and the X-ray detector 3 are during the measurement rotated at an angular velocity ratio of 1:2, and the angle of diffraction of the second monochromatic X-ray beams B6 and the wavelength thereof, both detected by the X-ray detector, provide an indication of the crystal structure of the specimen 2.
While it has been recognized that the composition of the specimen 2 more or less varies with a change in position of site of analysis, the above discussed prior art X-ray diffraction analysis has made it possible to analyze the specimen over a relatively large surface area, for example, 1 to 2 square centimeter.
However, since the first diffracted X-ray beams B2 diverge after having been converged at the X-ray collecting point Q, the X-ray path length inevitably becomes long. At the same time, the first diffracted X-ray beams B2 having been diverged travel not only in a direction parallel to the sheet of the drawing of FIG. 11, but also in a direction Z perpendicular to the sheet of the drawing of FIG. 11. Because of these reasons, the intensity of the X-ray beams eventually detected by the X-ray detector 3 tends to be lowered, resulting in a less accurate analytical measurement.
In any event, in this prior art optical system, the divergent angle .OMEGA.o of the diverging X-ray beams B1 generating from the X-ray radiation source P and subsequently impinging upon the curved analyzing crystal 50 is equal to the convergent angle .OMEGA. at which the first diffracted monochromatic X-ray beams B2 are converged at the X-ray collecting point Q. Because of this, in the case of the X-ray diffraction analysis wherein the first diffracted monochromatic X-ray beams B2 are required to be converged substantially parallel at a small convergent angle .OMEGA. before a specimen (not shown) is irradiated, the divergent angle .OMEGA.o of the X-ray beams B1 generating from the X-ray radiation source P must be small. Accordingly, since the intensity of the X-ray beams B1 impinging upon the analyzing crystal 50 decreases in proportion to the reduction of the divergent angle .OMEGA.o determined by the dimensions of the reflective surface 51, the intensity of the first diffracted monochromatic X-ray beams B2 ready to be incident upon the specimen tends to be lowered and, therefore, no reduction can be expected of the analytical error.
FIG. 12 illustrates the prior art total reflection fluorescence X-ray analysis apparatus. In this apparatus, X-ray beams B1 generating from the X-ray radiation source (X-ray source) P of an X-ray tube 5, travel towards a well-known Johanson monochromator crystal (analyzing crystal) 1A through a slit 5a. Characteristic X-ray beams of a predetermined wavelength contained in the X-ray beams B1 are diffracted by the monochromator crystal 1A to provide monochromatic X-ray beams (first-order X-ray beams) B2 which are subsequently impinge upon a surface 2a of the specimen 2 at a small incident angle .alpha., for example, 0.05 to 0.20 degree. A portion of the diffracted monochromatic X-ray beams B2 incident upon the specimen 2 undergoes a total reflection to provide reflected X-ray beams B4 while the remaining diffracted monochromatic X-ray beams B2 excite the specimen 2 to cause the latter to emit fluorescent X-ray beams B5 peculiar to analyzing elements contained in the specimen 2. The fluorescent X-ray beams B5 emitted from the specimen 2 are subsequently detected by an X-ray detector 3, disposed in face-to-face relationship with the surface 2a of the specimen 2. The X-ray detector 3 then determines the intensity of the fluorescent X-ray beams B5 and provides a detection signal a which is subsequently fed to a multi-pulse-height analyzer for providing X-ray spectra of interest.
In the above described total reflection fluorescence X-ray analysis, since the incident angle .alpha. of the diffracted monochromatic X-ray beams (first-order X-ray beams) B2 is very small, there is such an advantage that no substantial quantity of the totally reflected X-ray beams B4 as well as scattered X-ray beams enter the X-ray detector 4 and, therefore, as compared with an output level of the fluorescent X-ray beams B5 detected by the X-ray detector 3, a noise component is very small enough to provide a high signal-to-noise ratio (S/N ratio). For this reason, the sensitivity of analysis is so high as to make it available for the determination of a extremely small amount of impurities. In view of this, the total reflection fluorescence X-ray analysis is effectively and largely utilized for analyzing a surface contamination of silicon wafers.
In the practice of the prior art total reflection fluorescence X-ray analysis, since the analyzing crystal 1A is utilized to provide the primary monochromatic X-ray beams B2, the intensity of the scattering X-ray beams can be minimized to reduce the analytical error. On the other hand, however, if the X-ray beams B1 are rendered to be monochromatic, the intensity of the diffracted X-ray beams B2 is considerably lowered. This lowering of the intensity of the diffracted X-ray beams B2 is eliminated by the use of the curved analyzing crystal 1A so that the diffracted monochromatic X-ray beams B2 ace collected on the surface 2a of the specimen 2 to compensate for a reduction in intensity of the diffracted monochromatic X-ray beams B2 used to excite the specimen 2.
Unlike the standard fluorescence X-ray analysis apparatus, the prior art total reflection fluorescence X-ray analysis apparatus is ineffective to sufficiently increase the intensity of the diffracted monochromatic X-ray beams B2 used to irradiate the specimen even though the curved analyzing crystal 1A is employed. The reason therefor will now be discussed.
In this type of optical system, the divergent angle .OMEGA.o of the diverging X-ray beams B1 generated from the X-ray radiation source P and subsequently impinging upon the analyzing crystal 1A is equal to the convergent angle .OMEGA. at which the diffracted X-ray beams B2 are converged. On the other hand, in the total reflection fluorescence X-ray analysis, the incident angle .alpha. of the diffracted X-ray beams B2 upon the surface 2a of the specimen 2 is required to be small of 0.05 to 0.20 degree as hereinbefore described and, therefore, the divergent angle .OMEGA.o of the X-ray beams B2 diffracted from the analyzing crystal 1A must be chosen to be about 0.1 degree. This means that the divergent angle .OMEGA.o of the X-ray beams B1 before they are rendered to be monochromatic must be reduced to a very small value. Accordingly, the intensity of the diffracted monochromatic X-ray beams B2 ready to be incident upon the specimen tends to be lowered and, therefore, no satisfactory and sufficient reduction can be expected of the analytical error.